A transistor formed over a glass substrate or the like is manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in a liquid crystal display device. Although a transistor manufactured using amorphous silicon has low field-effect mobility, it has an advantage of being able to be formed over a larger glass substrate. On the other hand, although a transistor manufactured using polycrystalline silicon has high field-effect mobility, it has a disadvantage of not being suitable for a larger glass substrate.
In contrast to a transistor manufactured using silicon, a technique in which a transistor is manufactured using an oxide semiconductor and applied to an electronic device or an optical device has attracted attention. For example, Patent Document 1 and Patent Document 2 disclose a technique in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and such a transistor is used as a switching element or the like of a pixel of a display device.